Summary:
changed make unity target to use $LIB_SOURCES as a source of library source code. In the old way (using find) table/mock_table.h was added to a list of library objects and this was a course of `make unity` break.

`build_tools/unity` contains some redundant code, I deleted it and moved the functionality in the Makefile.

Test Plan:
Make sure unity completes with no errors.
```lang=bash
% make unity
```

Reviewers: sdong, rven, igor, meyering

Reviewed By: igor, meyering

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D35385